In recent years, portable terminals used for mobile communication have been increasingly reduced in size and lightened in weight. Along with this, the portable terminals are being made multi-banded to be able to handle a plurality of communication systems and are being given more functions and higher functions. Therefore, the built-in circuits have increased. For this reason, the electronic parts used are, for improvement of their mounting density, being strongly demanded to be surface mountable small-size parts.
One of the key parts of a portable terminal is an acoustic wave device. As the acoustic wave device, there are a surface acoustic wave device having an excitation electrode formed on a piezoelectric substrate or a piezoelectric thin film resonator sandwiching a thin piezoelectric film by metallic electrodes.
A surface acoustic wave device has to be provided with a hollow portion to secure a vibration space in a portion where an acoustic wave vibrates, and this vibration space has to be sealed air-tight. In such surface acoustic wave devices as well, together with low loss and an out-of-band cutoff characteristic, their being surface mountable small-sized surface acoustic wave devices is also being demanded.
In response to demands for smaller size, a surface acoustic wave device shown in FIG. 4 is proposed. In the surface acoustic wave device shown in FIG. 4, a vibration space of the surface acoustic wave is secured by forming a protective cover 46 so as to surround an IDT (inter digital transducer) formed on a piezoelectric substrate and exciting the surface acoustic wave (see, for example, Patent Document 1).
In this regard, cases employing, as electronic parts to be mounted in a portable terminal, electronic parts configured by a plurality of elements such as a surface acoustic wave device and IC or the like mounted on the same substrate, that is, formed into modules, are increasing. This is because by forming such modules, the circuit design efficiency can be improved. In addition, the mount circuit design is facilitated, the number of development steps is decreased, and the design period can be shortened. In electronic parts formed into modules, in order to protect the elements mounted on the same substrate, the elements are usually sealed by a resin by transfer molding. Note that, as electronic parts formed into modules which are mounted in a portable terminal, there are, for example, a GPS (Global Positioning System) module and a front end module processing transmitted and received signals.
In this regard, when resin-sealing the elements mounted on the substrate by transfer molding, a high pressure is applied to the elements under a high temperature atmosphere. In the conventional surface acoustic wave device shown in FIG. 4, a protective cover 46 was sometimes greatly deformed due to the high pressure at the time of the transfer molding. When the protective cover 46 was greatly deformed, contact of the protective cover 46 with the IDT electrodes, large distortion of the vibration space, or the like occurred thereby causing the problem of a large deterioration of electrical characteristics of the surface acoustic wave device.
Patent Document 1: Japanese Patent Publication (A) No. 9-246905